The prior art has included numerous methods for chemically "fixing" (i.e., tanning) biological tissues. Such chemical fixing of the biological tissues is often used as a means of preserving such tissues so that they may be used as, or incorporated into, bioprosthetic devices which are implanted in or attached to a patient's body. Examples of fixed biological tissues which have heretofore been utilized as bioprostheses include cardiac valves, blood vessels, skin, dura mater, pericardium, ligaments and tendons. These tissues typically contain connective tissue matrices which act as the supportive framework of the tissues. The cellular parenchyma of each tissue is disposed within and supported by it's connective tissue matrix.
Collagen and elastin are two substances which make up the connective tissue framework of most biological tissues. The pliability or rigidity of each biological tissue is largely determined by the relative amounts of collagen and elastin present within the tissue and/or by the physical structure and confirmation of the connective tissue frame work.
Each Collagen molecule consists of three (3) polypeptide chains which are intertwined in a coiled helical confirmation. Chemical fixatives (i.e., tanning agents) used to preserve collagenous biological tissues generally form chemical cross-linkages between the amino groups on the polypeptide chains within a given collagen molecules, or between adjacent collagen molecules.
The chemical cross-linkages formed between polypeptide chains within a single collagen molecule are termed "intramolecular", while the cross-linkages formed between polypeptide chains of different collagen molecules are termed "intermolecular".
Chemical fixative agents which have been utilized to cross-link collagenous biological tissues include; formaldehyde, glutaraldehyde, dialdehyde starch, hexamethylene diisocyanate and certain polyepoxy compounds.
In particular, glutaraldehyde has proven to be a suitable agent for fixing various biological tissues used for subsequent surgical implantation. Indeed, glutaraldehyde has become widely used as a chemical fixative for many commercially available bioprostheses, such as; porcine bioprosthetic heart valves (i.e., the Carpentier-Edwardss stented porcine bioprosthesis; Baxter Healthcare Corporation; Edwards CVS Division, Irvine, Calif. 92714-5686), bovine pericardial heart valve prostheses (e.g., Carpentier-Edwards SPericardial Bioprosthesis, Baxter Healthcare Corporation, Edwards CVS Division; Irvine, Calif. 92714-5686) and stentless porcine aortic prostheses (e.g., Edwardss PRIMAM Stentless Aortic Bioprosthesis, Baxter Edwards AG, Spierstrasse 5, GH6048, Horn, Switzerland).
One problem associated with the implantation of bioprosthetic materials is that collagen and elastin typically contained in these materials tend to undergo calcification. Such calcification can result in undesirable stiffening or degradation of the bioprosthesis. Both intrinsic and extrinsic calcification are known to occur in fixed collagenous bioprostheses, although the exact mechanism(s) by which such calcification occurs is unknown.
Clinical experience and experimental data has taught that glutaraldehyde-fixed collagenous bioprostheses may tend to calcify sooner than bioprostheses which have been fixed by other nonaldehyde fixative agents. Such accelerated calcification of glutaraldehyde-fixed bioprostheses has been reported to occur most predominantly in pediatric patients. (Carpentier et al., Continuing Improvements in Valvular Bioprostheses, J. Thorac Cardiovasc. Surg. 83:27-42, 1982.) Such accelerated calcification is undesirable in that it may lead to deterioration and/or failure of the implanted bioprostheses. In view of this propensity for accelerated calcification of glutaraldehyde-fixed bioprostheses in young patients, surgeons typically opt to implant mechanical heart valves or homografts (if available) into pediatric or relatively young patients (i.e., patients under 65 years of age), rather than glutaraldehyde-fixed bioprosthetic valves. However, patients who receive mechanical valve implants require ongoing treatment with anticoagulant medications, which can be associated with increased risk of hemorrhage. Also, homografts are of limited availability and may carry pathogens which can result in infection.
The factors which determine the rate at which glutaraldehyde-fixed bioprosthetic grafts undergo calcification have not been fully elucidated. However, factors which are thought to influence the rate of calcification include:
a) patient's age; PA1 b) existing metabolic disorders (i.e., hypercalcemia, diabetes, etc.); PA1 c) dietary factors; PA1 d) race; PA1 e) infection; PA1 f) parenteral calcium administration; PA1 g) dehydration; PA1 h) distortion/mechanical factors; PA1 i) inadequate coagulation therapy during initial period following surgical implantation; and PA1 j) host tissue responses. PA1 a) providing a collagenous bioprosthesis which has been cross-linked with glutaraldehyde; PA1 b) reacting at least some of the carboxyl groups present on collagen molecules of the bioprosthesis with a carboxyl activating agent to convert at least some of the carboxyl groups into activated carboxyl moieties; PA1 c) reacting a carboxyl-free compound with said activated carboxyl moieties, thereby forming carboxyl-free side groups on the collagen molecules of the bioprosthesis. PA1 d) contacting the bioprosthesis with glutaraldehyde.
Many investigators have attempted to discover ways of mitigating the in situ calcification of glutaraldehyde-fixed bioprostheses. Included among these calcification mitigating techniques are the methods described in U.S. Pat. No. 4,885,005 (Nashef et al.) entitled Surfactant Treatment of Implantable Biological Tissue To Inhibit Calcification; U.S. Pat. No. 4,648,881 (Carpentier et al.) entitled Implantable Biological Tissue and Process For Preparation Thereof; U.S. Pat. No. 4,976,733 (Girardot) entitled Prevention of Prosthesis Calcification; U.S. Pat, No. 4,120,649 (Schechter) entitled Transplants; U.S. Pat. No. 5,002,2566 (Carpentier) entitled Calcification Mitigation of Bioprosthetic Implants; EP 103947A2 (Pollock et al.) entitled Method For Inhibiting Mineralization of Natural Tissue During Implantation and WO 84/01879 (Nashef et al.) entitled Surfactant Treatment of Implantable Biological Tissue to Inhibit Calcification; and, in Yi, D., Liu, W., Yang, J., Wang, B., Dong, G., and Tan, H.; Study of Calcification Mechanism and Anti-calcification On Cardiac Bioprostheses Pgs. 17-22, Proceedings of Chinese Tissue Valve Conference, Beijing, China, June 1995.
There remains a need in the art for the development of new methods for inhibiting or mitigating calcification of glutaraldehyde-fixed biological tissues.